There are two types of receptors for the neurotransmitter, acetylcholine: muscarinic receptors and nicotinic receptors, based on the selectivity of action of muscarine and nicotine, respectively. Muscarinic receptors are G-protein coupled receptors. Nicotinic receptors are members of the ligand-gated ion channel family. When activated, the conductance of ions across the nicotinic ion channels increases.
Nicotinic alpha-7 receptor protein forms a homo-pentameric channel in vitro that is highly permeable to a variety of cations (e.g., Ca++). Each nicotinic alpha-7 receptor has four transmembrane domains, named M1, M2, M3, and M4. The M2 domain has been suggested to form the wall lining the channel. Sequence alignment shows that nicotinic alpha-7 is highly conserved during evolution. The M2 domain that lines the channel is identical in protein sequence from chicken to human. For discussions of the alpha-7 receptor, see, e.g., Revab et al. (1991), Nature, 353, 846-849; Galzi et al. (1992), Nature 359, 500-505; Fucile et al. (2000), PNAS 97(7), 3643-3648; Briggs et al. (1999), Eur. J. Pharmacol. 366 (2-3), 301-308; and Gopalakrishnan et al. (1995), Eur. J. Pharmacol. 290(3), 237-246.
The nicotinic alpha-7 receptor channel is expressed in various brain regions and is believed to be involved in many important biological processes in the central nervous system (CNS), including learning and memory. Nicotinic alpha-7 receptors are localized on both presynaptic and postsynaptic terminals and have been suggested to be involved in modulating synaptic transmission. It is therefore of interest to develop novel compounds, which act as ligands for the α7 nAChR subtype, for the treatment of disease conditions associated with defective or malfunctioning nicotinic acetylcholine receptors.